The present invention relates to radio communication systems. In particular, the present invention is related to communication systems which use frequency hopping in un-licensed frequency carriers.
In the last decades, progress in radio and VLSI technology has fostered widespread use of radio communications in consumer applications, portable devices, such as mobile radios, can now be produced having acceptable cost, size and power consumption.
In the system described in U.S. patent application Ser. No. 09/385,024, supra (hereinafter “the parent application”), a hybrid or combined communication channel is used which includes a Frequency Hopping (FH) channel for medium-range, low-rate services and a static or fixed frequency channel for short-range, high-rate services. The system may be considered generally based on the Bluetooth air interface which defines a FH channel using a basic rate of 1 Mb/s in the 2.4 GHz ISM band. For more information on Bluetooth, see, for example, “Bluetooth, the Universal Radio Interface for Ad Hoc wireless connectivity”, J. C. Haartsen, Ericsson Review, Telecommunications Technology Journal, No. 3, 1998. In a Bluetooth piconet, for example as described in the parent application, a high rate static channel may be defined by selecting a broadband channel, e.g. 4 MHZ, which may be positioned at a spectral location which may be determined to have the least amount of interference. The static channel may be determined by adaptive channel allocation based on receive signal strength measurements carried out by the radio units. Once a static channel of about 4 MHZ bandwidth has been selected in the manner described, high data rate communication, for example, between multiple slave may proceed directly over the fixed channel.
Communication over the FH channel is configured to avoid the frequency band occupied by the static channel in order to avoid mutual interference. The parent application describes how the FH sequence used on the FH channel may be adapted to avoid the frequency band occupied by the static high-rate channel. It should be noted that since a typical Bluetooth-based system uses 79 channels, and since only 75 are generally required, the FH sequence can be adapted to avoid 4 carriers of 1 MHZ each. The combined bandwidth of the avoided channels represents 4 MHZ, thus fitting the requirement for the high-rate channel.
It should further be noted that the FCC rules require systems to spread signal energy when operating in the 2.4 GHz ISM band at power levels higher than 0.75 mW but lower than 1 W. Frequency hopping systems are further required to hop over at least 75 carriers, with the hop channel bandwidth restricted to 1 MHZ. Aside from signal energy spreading from frequency hopping, signal energy spreading may also be obtained using direct-sequence spread spectrum (DSSS), provided the spreading gain is at least 10. For systems having higher user rates, DSSS may be less appropriate since the requirement for broader bandwidth increases the probability that a DSSS system may interfere with or be interfered by another system operating in the ISM band.
On the high-rate 4 MHZ fixed channel a multilevel modulation is proposed as described in the parent application, with the relatively narrowband fixed channel providing about 10 Mb/s of user rate. The narrowband fixed channel allows much of the ISM band to be rejected by filtering; and thus the overlap probability is reduced. However, since the DSSS rules which specify, inter alia, 10 dB processing gain, and the FHSS rules which specify, inter alia, no hop channel bandwidth larger than 1 MHZ, are unfulfilled by the multilevel modulation scheme, data transfer on the high-rate fixed channel operates under the low-power rules, e.g. transmit power of 0.75 mW or lower, thus limiting the range associated with the fixed channel.
Accordingly, requests have been made by several manufacturers of FH systems operating in the ISM band to relax the requirements on the FH channel. In particular, the bandwidth restriction of 1 MHZ restricts the data rate in FH systems to 1–2 Mb/s.
It would therefore be appreciated that a need exists in the art for a method and apparatus for providing an increase in frequency hopping bandwidth and an increase in transmit power in accordance with revised ISM requirements.